Interpretation of the MSDP Hα line profiles recorded during the gradual phase of a solar flare

Abstract

In our paper we presented the results of analysis of Hα line profiles emitted by some bright flaring structures observed during the gradual phase of the C7.2 X-ray class flare. The flare was observed in NOAA 8088 active region on 25 September 1997 at 11:50 UT. We have used Hα line spectra obtained with the Wrocław Multichannel Subtractive Double Pass Spectrograph for all the points of the recorded field of view with the temporal resolution of the order of 2 minutes. We analysed the time evolution of the Hα profiles emitted by some selected structures of the flare. We found that during the gradual phase of the flare the Hα spectra exhibited mainly redshifts. The line of sight velocities, corresponding to these shifts, did not exceed +13 km s −1. We found that only few very small areas outside the flare structures showed blueshifted spectra. Hα emission kernels located probably close to X-ray loops footpoints also exhibited redshifted line profiles. In contrast, the modelling made by Antiochos and Sturrock (1978) as well as by Acton et al. (1982) predict the existence of a “gentle evaporation” of plasma heated during the thermal phase of the flares. Such a gentle upward motion of the chromospheric plasma should produce mostly blueshifted profiles of the observed chromospheric spectral lines (Schmieder et al., 1987).